Circuit breakers for low voltages are known and are used as protection switches in installations for electrical power distribution. A conductor runs through the switch for each current phase, and an alternating current flows through each of these conductors. When the switch is closed, contact elements of the switch make contact with one another. A current transformer is in each case arranged directly on the conductor of each current phase in the switch and emits an electrical signal, which is dependent on the current flowing and is evaluated. The current detected in this way is compared by an overcurrent release (tripping unit) with a current limit value, and the overcurrent release trips the switch if this current limit value is exceeded. Tripping occurs in particular when a long-lasting overcurrent or short-circuit current occurs. The current limit value is dependent on parameters and characteristics.
In order to open the switch, the switching contacts are opened with the aid of a tripping shaft, which is held prestressed by a switching mechanism. The switching mechanism is unlatched by a tripping magnet (electromagnet), which has a winding through which a current is sent in the event of tripping. Unlatching the switching mechanism releases the mechanical energy which is stored in a spring energy store. The stored energy is released by a tripping magnet with relatively little force. The tripping magnet is operated by the overcurrent release, and the switching mechanism is unlatched by means of the movable tripping magnet (plunger).
The operation of the switch is therefore directly dependent on the operation of the tripping magnet. Failure of the tripping magnet to operate because of a broken wire or mechanical blocking may have serious consequences.